Considering that you have run a AD synchronization with your UPSA and that your AD connection rules include the OU of the user you are querying for, you should be able to retrieve the specific AD fields (in this case email) using JSOM. Check SP.Userprofiles (https://msdn.microsoft.com/en-us/library/office/jj920104.aspx)
var userProfileProperties;
// Ensure that the SP.UserProfiles.js file is loaded before the custom code runs.
SP.SOD.executeOrDelayUntilScriptLoaded(getUserProperties, 'SP.UserProfiles.js');
function getUserProperties() {
// Replace the placeholder value with the target user's credentials.
var targetUser = "domainName\\userName";
// Get the current client context and PeopleManager instance.
var clientContext = new SP.ClientContext.get_current();
var peopleManager = new SP.UserProfiles.PeopleManager(clientContext);
// Specify the properties to retrieve and target user for the
// UserProfilePropertiesForUser object.
var profilePropertyNames = ["Email"];
var userProfilePropertiesForUser =
new SP.UserProfiles.UserProfilePropertiesForUser(
clientContext,
targetUser,
profilePropertyNames);
// Get user profile properties for the target user.
// To get the value for only one user profile property, use the
// getUserProfilePropertyFor method.
userProfileProperties = peopleManager.getUserProfilePropertiesFor(
userProfilePropertiesForUser);
// Load the UserProfilePropertiesForUser object and send the request.
clientContext.load(userProfilePropertiesForUser);
clientContext.executeQueryAsync(onRequestSuccess, onRequestFail);
}
// This function runs if the executeQueryAsync call succeeds.
function onRequestSuccess() {
var messageText = "\"Email\" property is "
+ userProfileProperties[0];
$get("results").innerHTML = messageText;
}
// This function runs if the executeQueryAsync call fails.
function onRequestFail(sender, args) {
$get("results").innerHTML = "Error: " + args.get_message();
}